Eye On Electronics

Motor, Sep 2007 by Dale, Mike

Vehicle safely and convenience features employing advanced electronics continue to be introduced. Understanding the science behind these systems will give you a leg up if problems develop.

You may have heard of J.D. Power and Associates, but you may not be aware of the great influence its customer satisfaction surveys have on the design, manufacturing and sale of automobiles. Their initial quality surveys and other consumer research into buyer satisfaction are used as report cards by the industry. Manufacturers take these lists and use them to focus on areas of their designs that need improvement.

"Windows fog up a lot" was ranked as the second highest issue in one J.D. Power quality survey for HVAC systems. A separate study done by a group called Allison Fisher said that even at a projected added cost of $75, automatic defogging was as highly desired by consumers as such features as remote starting and steering wheel radio controls. The industry response to tiiis has been an increasingly sophisticated approach to automatic windshield fogging control.

While the causes of fog, mist and/or frost on a windshield are well known, a good solution has been more elusive. This is an area where driver behavior, including ignorance of how to operate the system, may be as much of an issue as system performance. You could look at the automatic system as being a way to prevent fogging that doesn't include educating the driver.

The basic science of fog on a windshield is pretty straightforward. Cold windshields fog up because they're in contact with air that has more water vapor content than it can hold, relative to the temperature of the windshield. The term dew point is used to describe the temperature level at which the humidity in the air condenses into a liquid because the air can no longer hold the moisture, in the form of water vapor.

Two factors-the presence of water vapor in the air and cold surface temperatures-are the key elements to the formation of fog or frost on a windshield. Clearing the windshield or, better yet, preventing it from fogging in the first place, depends on interrupting one or botii of these two issues. Given the amount of glass on a typical vehicle, either heating all of the windows or removing all of the water vapor from the vehicle's interior takes some doing.

Heating the windows so they do not represent a cold surface to the cabin air can be done in a couple of ways. Stationary rear windows can be warmed by electric grids embedded between the layers of glass or applied to the inside surface. This technique is widely used. The grids cannot be used in movable side windows due to the needed electrical connections. They're also not appropriate for the front windshield, as they'd interfere with the driver's vision.

Ford, Lexus and others have tried a metallization in the front windshield that was supposed to be able to heat the glass witiiout limiting the driver's vision. Ford eventually dropped this idea. For one tiling, making the gíass turned out to be very expensive, adding hundreds of dollars to the purchase price of the vehicle. Every time a passing gravel truck lost another piece of its load, it was sometiiing of a financial disaster for the driver of the vehicle behind it or his insurance company to replace it. Probably even more important, when it was needed most, during cold startup, the required amperage to run the heater was a heavy draw on the alternator and battery. Since this load typically occurs at the same time other major electrical loads such as the wipers and HVAC blower are also likely to be in use, it was a real strain on the system. This explains why Ford employed a system that directed the full unrectified output of the alternator to the windshield in an effort to get enough power to melt frost.

The alternative is to heat the windows with air directed from the HVAC system, but this concept is limited in several ways. The first is that for the first minutes of operation, there's little or no heat in the cooling system that can be directed at the windows. The second is that vents directing air are often too far away from where the warm air is needed most. It can be a long way from the base of the windshield to the top and outer edges. Side windows and rear windows may never really see enough warm air to clear them. Yet another issue is that of suddenly changing conditions. A sudden downpour can change the windshield temperature faster than the system can hope to keep up with.

Another area of opportunity to combat windshield fogging is in controlling the humidity of the air inside the passenger cabin. If the humidity could be eliminated or reduced, there wouldn't be as much of a need to heat the windows. Ambient humidity is a function of local weather and seasonal conditions. Simply put, Tucson has some advantages over New Orleans when it comes to keeping car windows fog-free.

One source of bodi moisture creation and moisture removal is the HVAC system itself. Blowing warm, humid air from the passenger compartment over the cold evaporator results in some of that moisture condensing onto the evaporator. Dehumidifying the air actually adds to passenger comfort inside of the vehicle by improving the evaporation of the passenger's own personal cooling system-perspiration.